Fischer-Tropsch synthesis is a known reaction for the production of hydrocarbons from syngas (a mixture of carbon monoxide and hydrogen), wherein syngas is contacted with a heterogeneous catalyst to produce a mixture of hydrocarbons. Syngas is typically produced by processes such as steam reforming of coal or natural gas, or from the partial oxidation of natural gas, and it may also be produced from biomass. One application of Fischer-Tropsch synthesis is in the production of hydrocarbon liquids and/or waxes that may be used as fuels or in the production of fuels through processes such as hydrocracking.
During heterogeneously catalysed processes for Fischer-Tropsch synthesis of hydrocarbons, product hydrocarbons that are liquid or solid under reaction conditions can condense on the catalyst surface, which inhibits contact of the syngas reactant with the catalyst surface and results in reduced conversion of reactants.
Variable diameter reactors have hitherto been described for controlling reaction temperatures in processes involving reactants and products that are in the gas-phase under reaction conditions. Thus, WO 03/011449 describes an apparatus in which the cross-sectional area of a solid catalyst bed is increased along its longitudinal axis by use of shaped inserts carrying heat transfer material, and DE 2 929 300 describes a variable diameter reactor for controlling the temperature of catalyst in endothermic or exothermic reactions in which the shape of inserts carrying heat-transfer material is varied along their length. However, the processes described therein do not produce products that are in a condensed-phase under reaction conditions, and hence do not address the issue of coverage of the solid catalyst with condensed-phase product.